Some vehicles to be used as construction machines such as wheel loaders and bulldozers include a hydraulic drive apparatus called Hydro-Static Transmission (HST) between the engine as the drive source and driven wheels. A hydraulic drive apparatus includes a variable displacement hydraulic pump that is driven by an engine, and a hydraulic motor of a variable capacity type that is driven by the pressure oil discharged from the hydraulic pump. The hydraulic pump and the hydraulic motor are provided in a main hydraulic circuit that is a closed circuit. The hydraulic drive apparatus transmits the drive force generated from the hydraulic motor to the driven wheels, so as to move the vehicle.
In a vehicle that is equipped with this hydraulic drive apparatus, the capacity of the hydraulic pump and the capacity of the hydraulic motor are appropriately adjusted, so as to arbitrarily change the revolution speed ratio between the hydraulic pump and the hydraulic motor. Accordingly, the speed of the vehicle can be changed in a non-step fashion simply by pressing down the accelerator pedal without a complicated lever operation, and the operability can be dramatically increased.
In this type of hydraulic drive apparatus, the pressure loss is normally reduced by lowering the pressure of the main hydraulic circuit at the time of driving, and the hydraulic efficiency is made higher accordingly. For example, in a hydraulic drive apparatus that controls a tilt angle of the hydraulic pump in accordance with the pilot pressure from the charge pump, a cutoff valve is provided in the middle of the oil passage for supplying the pilot pressure. In such a hydraulic drive apparatus, the pilot pressure is reduced when the pressure of the main hydraulic circuit becomes equal to or higher than a cutoff pressure that is set in the cutoff valve, so as to reduce the tilt angle of the hydraulic pump. In this manner a discharge amount of the pressure oil supplied from the hydraulic pump to the main hydraulic circuit is reduced (see Patent Document 1, for example).
In some hydraulic drive apparatuses that are equipped with HST, an inching valve is provided in the oil passage for supplying the pilot pressure. In such a hydraulic drive apparatus, the pilot pressure is reduced when an operator operates an inching pedal, so as to reduce the tilt angle of the hydraulic pump. In this manner, the discharge amount of the pressure oil supplied from the hydraulic pump to the main hydraulic circuit can be restricted.    Patent Document 1: JP-A-2004-232469 (FIG. 1)